1. Field of the Invention
The present invention relates to a method of allocating radio channels for a cellular radio communication system in which each base station covers a plurality of sectoral zones with a plurality of antennas each having a sectoral directivity pattern in a horizontal plane.
2. Description of the Prior Art
Mobile communication systems such as automobile telephone systems employ the same channel repeatedly in zones which are free from mutual interference for effective utilization of frequencies. There are available two channel allocation methods, i.e., the fixed channel allocation method and the dynamic channel allocation method. According to the fixed channel allocation method, specified channels are fixedly allocated to each zone in view of interference conditions. According to the dynamic channel allocation method, channels are not allocated to each zone in a fixed manner, but a base station selects one of all channels per communication request, measures the carrier-to-interference power ratio (hereinafter referred to as "CIR") of the selected channel, and allocates the selected channel if the measured CIR is higher than a predetermined threshold value (hereinafter referred to as "CIR threshold value"). With the dynamic channel allocation method, channels can effectively be utilized by a jumbo group effect that is achieved when all base stations share and allocate all channels. Further, since a channel can repeatedly be used if the CIR threshold value is satisfied, the dynamic channel allocation method can realize a shorter distance of repeated use of the same channel than the fixed channel allocation method. Therefore, the dynamic channel allocation method has a higher frequency utilization efficiency than the fixed channel allocation method.
Zones of mobile communication systems may be arranged in either an omnidirectional zone configuration or a sectoral zone configuration. In the omnidirectional zone configuration, a base station has an antenna having an omnidirectional directivity pattern in a horizontal plane, and one zone around one antenna is covered by one base station. In the sectoral zone configuration, a base station has a plurality of antennas each having a sectoral directivity pattern in a horizontal plane, and a plurality of sectoral zones are covered by this base station. Since the same channel interference is low due to the antenna directivity, and the distance for repeatedly using the same channel is shorter than in the omnidirectional zone configuration, the sectoral zone configuration has a higher frequency utilization efficiency than the omnidirectional zone configuration.
Heretofore, the sectoral zone configuration has been used in combination with the fixed channel allocation method. Channel allocation patterns available for this combination include the parallel beam channel allocation pattern and the back-back beam channel allocation pattern. See, for example, "Automobile Telephone" by Moriji Kuwabara, Electronic Information Communication Society, 1985, pages 79.about.83. FIG. 12(a) of the accompanying drawings shows the parallel beam channel allocation pattern. In FIG. 12(a), there are three base stations A, B, C, and the same channel is allocated to sectoral zones A1, C1 of the same direction which are covered by base stations A, C spaced a certain distance from each other. FIG. 12(b) of the accompanying drawings shows the back-back beam channel allocation pattern. In FIG. 12(b), there are three base stations A, B, C, and the same channel is allocated to sectoral zones A1, B1, C1 which extend in directions opposite to the directions to the specified area.
According to the parallel beam channel allocation pattern, since the same channel is used in those sectoral zones whose antenna directivity is in the same direction, it is highly probable that a major interference wave will arrive in the same direction as that of a desired carrier wave. In actual propagation environments, since major factors which determine variations of the central values of carrier and interference waves are ground configurations and objects in the direction in which the carrier and interference waves arrive, the carrier and interference waves are highly correlated to each other if they arrive in the same direction (see "Propagation Correlations at 900 MHz" by V. Graziano, IEEE Trans. on Vehicular Technology VT-27, No. 4, November, 1978). Accordingly, in the parallel beam channel allocation pattern, where the level of a carrier wave is low, it is highly probable that the level of an interference wave is also low, with the result that the CIR may not be readily decreased. Further, since interference in the direction perpendicular to the antenna directivity is small, the CIR may not be readily decreased.
The back-back beam channel allocation pattern is advantageous if regions of large traffic are limited, because the distance for repeatedly using the same channel is locally shortened.
While the sectoral zone configuration has heretofore been used in combination with the fixed channel allocation method, as described above, it has also been proposed to use the sectoral zone configuration in combination with the dynamic channel allocation method for much higher frequency utilization efficiency (see "Adaptive Channel Allocation in a TIA IS-54 System" by H. Andersson, H. Eriksson, A. Fallgren, and M. Madfors, 1992 Vehicular Technology Conference, pages 778.about.781).
For dynamic channel allocation in the omnidirectional zone configuration, a channel is selected from all channels and allocated per base station. For dynamic channel allocation in the sectoral zone configuration, it may be possible to select a channel from all channels and allocate the selected channel per sectoral zone. The combination of the sectoral zone configuration and the dynamic channel allocation method would achieve the advantage of reduced interference in the same channel due to the sectoral zone configuration and the advantage of the dynamic channel allocation method but would fail to obtain the advantages of the parallel beam channel allocation pattern and the back-back beam channel allocation pattern.